The diagnosis of ductal carcinoma in situ (DCIS) of the breast has increased 400-500% since 1980, and DCIS now comprises up to 20% of all breast cancers diagnosed in screened populations. Approximately 10-20% of women treated with breast-conserving surgery will have a subsequent ipsilateral breast cancer within 5 years of a diagnosis of DCIS; however, few factors have been established that identify those women who are most likely to experience recurrent disease. Mammographic density is one of the strongest known risk factors for breast cancer, yet it has been described as among the most undervalued and underutilized factors in breast cancer studies. Most breast cancer recurrences following DCIS are located in the involved, ipsilateral breast. The extent to which the ipsilateral breast is occupied by radiologically dense tissue at diagnosis may reflect the activity of hormones and other factors capable of stimulating the growth and proliferation of occult malignant cells that are often left behind after breast-conserving surgery for DCIS. The aim of this cohort study if to determine whether mammographic features of the breast present at diagnosis of DCIS predict risk of subsequent ipsilateral breast cancer. The study will include all women in a defined population, the Northern California Kaiser Permanente Medical Care Program, between the ages of 20 and 74 who were diagnosed with a first primary DCIS from 1990 to 1997 and treated with breast-conserving surgery. The cohort will include approximately 1,200 women with DCIS who will be followed for ipsilateral breast cancer by reviewing medical records and information in computerized databases. The mammographic films from the primary DCIS will be retrieved and blindly evaluated by experts for microcalcifications, parenchymal pattern, and total area and percent of the breast occupied by radiological dense tissue. Medical records will also be reviewed to collect information on potentially confounding factors, such as body weight and hormone use. The proposes study will advance our understanding of factors that are related to prognosis among women with DCIS and provide information that may assist patients and their doctors with treatment decisions. In the future, knowledge about mammographic features may also help target those women with DCIS most likely to benefit from therapies, such as tamoxifen, that are mediated through a process reflected directly or indirectly by mammographic density.